The present invention relates to a thread tensioning device for a sewing machine. For well-known thread tensioning devices used in the conventional sewing machines it is next to impossible to automatically change the tension imparted to the thread leading from a supply of thread to a stitch-forming instrumentality, such as a needle, when the size or thickness of the thread is changed on replacement thereof. For forming a good-looking stitch wherein upper and lower threads are so well balanced in tension that the concatenation thereof is set at the midway point between the upper and lower faces of the work pieces, the tensioning device must be manually operated to appropriately adjust the tension to be imparted to the thread whenever the same is replaced.
The adjustment of tension inevitably requires a trial sewing, which is time-consuming even for skilled operators, and is cumbersome for unskilled beginners beyond description. Simplification of this tension adjustment, which will naturally lead to a simplification of machine operation and forming of good-looking stitches, has been an essential problem in this field to be solved yet.